1. Field of the Invention
This invention relates generally to a shelf assembly in an electronic switching system, and particularly to a storage structure for storing cables in a shelf assembly.
2. Description of the Related Art
With the progress of higher density of electronic switching systems in recent years, easiness and rapidness of maintenance/services are demanded with respect to subscriber's lines. In order to meet with this demand, there is a tendency to individualize the subscriber's lines (i.e., "per-line") in which each subscriber's line is stored in a single package so that only that particular package may be removed for repair in case of a failure or fault in order not to affect the remaining subscriber's lines.
With the progress of the "per-line" of the subscriber's lines, there is proposed a shelf assembly in which a plurality of drawer units having a plurality of "per-line" packages each storing the contents of service per subscriber removably mounted therein, are withdrawably placed on the shelf. Further placed on the shelf is a common unit which is to be interfaced with other systems. The common unit and the drawer unit are connected to each other through a plurality of cables. However, this involves the problems that as the result of "per-line" of the packages, the amount of cable required is increased, cable formability is deteriorated in assembling factories, and a cooling of the system is interrupted by the cables which altogether serve as an interruption wall.
Heretofore, there is employed a construction in which the cables coming out of the common unit are connected respectively to the backs of the drawer units. According to this cable connection structure, the cables are formed at an assembling factory such that each cable is in a generally fully stretched condition when each drawer unit has been withdrawn fully. When each drawer unit is inserted in the shelf, each cable is stored in the shelf in its bending condition but with a part of the cable slightly projecting from the back of the shelf. The switching system comprises a plurality of such shelf assemblies placed on a rack.
However, in the conventional storage structure having the cables stored in the shelf, since the cables connected to the backs of the drawer units are left free without being supported by anything within the shelf, there is a possibility that the cables are accidentally caught by the shelf when the drawer units are pushed into the shelf. Also, there is a possibility that the cables accidentally enter into the insertion area of the adjacent drawer unit to contact the package, etc. inserted in the drawer unit, and move together with them. As a result, a rapture, etc. are likely to occur to the outer jackets of the cables thereby adversely affecting the durability and reliability of the switching system.
Since the cables partly project from the back of the shelf, it becomes impossible to carry out a maintenance from the back in the event the switching system comprises the shelf assemblies which are placed in back-to-back relation on the racks. This means that a lot of processes are required when the cables have been subject to failure.
In the case where the shelf assemblies are placed in back-to-back relation, a ventilation path is provided between the back-to-back shelf assemblies. However, since the cables partly project from the backs of the shelves, heat generated from heating components located below becomes impossible to ascend in a satisfactory manner, thus resulting in significant reduction of cooling capacity of the switching system.
Furthermore, since a gap or space between the adjacent drawer units is small in the conventional shelf assembly, the heat of the packages inserted in the drawer units tends to flow into the adjacent drawer unit before its temperature is satisfactorily lowered. This can again cause the cooling capacity of the switching system to be lowered.